Environmental Matters - Accrued Liabilities and Regulatory Assets (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Accrued Liabilities:
Accrued Liabilities	$ 767	$ 765
Regulatory assets	8,082	8,228
Manufactured gas plant sites
Accrued Liabilities:
Accrued Liabilities	678	679
Other Superfund Sites
Accrued Liabilities:
Accrued Liabilities	89	86
Superfund Sites
Accrued Liabilities:
Accrued Liabilities	767	765
Regulatory assets	851	904
CECONY
Accrued Liabilities:
Accrued Liabilities	667	665
Regulatory assets	7,475	7,603
CECONY | Manufactured gas plant sites
Accrued Liabilities:
Accrued Liabilities	578	579
CECONY | Other Superfund Sites
Accrued Liabilities:
Accrued Liabilities	89	86
CECONY | Superfund Sites
Accrued Liabilities:
Accrued Liabilities	667	665
Regulatory assets	$ 746	$ 800